Islamic State has gained momentum in Syria, boosted by equipment seized in rapid offensive in neighbouring Iraq

BEIRUT, Aug 17 (Reuters) - Forces loyal to President Bashar al-Assad hammered the eastern Syrian city of Raqqa with more than two dozen air strikes on Sunday, targeting areas controlled by the Islamic State militant group, a monitoring group said.

The Syrian Observatory for Human Rights, which has tracked violence on all sides of the conflict that began in March 2011, said at least 31 fighters from the Islamic State were killed and dozens wounded in the air strikes that hit Raqqa city and the surrounding areas.

It said 26 strikes on Sunday hit Islamic State buildings, including the military court and bases in the city.

The conflict in Syria started when Assad cracked down on a pro-democracy uprising, which then armed itself.

Until this summer, Assad's forces held off from targeting the al Qaeda offshoot, formerly known as the Islamic State in Iraq and the Levant (ISIL). This has allowed the group to thrive and also weaken less hardline opposition groups that are backed by the West.

Assad has long painted the uprising in Syria as a foreign-backed Islamist conspiracy and his enemies say he has allowed the Islamic State to grow to promote that idea.

But this month, Islamic State fighters have gained momentum in Syria, boosted by equipment seized in a rapid offensive in neighbouring Iraq, and the Syrian army has become more confrontational, using air strikes to kill fighters.

Raqqa is a major stronghold of the Islamic State, which took one of the Syrian government's last outposts there on Thursday.

A resident in Raqqa said government warplanes had stepped up their bombardment five days ago.

On Saturday, there had been 16 air strikes, with dozens of artillery rounds also fired at areas under Islamic State control, he said, speaking via Skype and declining to be identified out of concern for his safety.

"About 30 percent of the strikes hit the Islamic State positions, the rest (hit) civilian areas," he said.

More than 170,000 people have been killed in Syria's civil war, which pits overwhelmingly Sunni Muslim rebels against Assad, a member of the Shi'ite-derived Alawite minority, backed by Shi'ite militias from Iraq and Lebanon. (Reporting by Oliver Holmes and Tom Perry; Editing by Alison Williams)